It is known to provide solar-energy collectors with flow chambers traversed by an energy-absorbing fluid, a gas-containing heat-insulating space between the flow chamber and the source of solar energy, i.e. the sun, and a frame supporting this structure.
Generally the frame carries an outer covered glass pane which defines the outer insulating chamber and is provided with fittings which connect the flow chamber or channels into the circulating line for the fluid.
The heat-insulating chamber can be filled with air or another gas or a gas mixture and is generally under at most ambient (atmospheric) pressure although it may also be under a subatmospheric pressure to reduce convective heat loss.
The fluid is a heat exchange medium which can be selected so that it is highly heat-absorbent itself or may merely take up heat by heat exchange with the heat-absorbing surfaces of the flow chamber.
The flow chamber thus functions also as a heat exchanger in which the incident solar radiation is transformed into sensible heat energy which, in turn, is transferred to or picked up by the fluid.
When reference is made herein to glass panes, it should be understood that transparent plates or sheets are intended and that these can be also composed of synthetic-resin material.
A solar collector of the aforedescribed type is described in publication P.B.-253 150, 1976, page 64, U.S. Department of Commerce, National Technical Information Service, the solar-energy collector being provided in the flow chamber with flow passages or channels. In this construction the flow chamber is a separate structure requiring especially expensive fabrication techniques.
In other fields it is known to provide, for acoustic insulation and/or thermo-insulation purposes, insulating glass units, i.e. so-called double-pane units, which consist of inner and outer panes defining a dead-air space between them and sealing means surrounding the space and holding the panes apart. The space may contain air or another gas filling, may be evacuated or may be under reduced pressure.
In all such units, however, the two panes are sealed directly together or are sealed together via the aforementioned sealing means.
The spacer frame can be a separate element or can be formed by bending the glass panes at their perimeters. In the latter case, the bent flanges of a glass pane may be sealed to the opposing glass pane to form the hermetically sealed unit.
In the glass industry, the fabrication techniques for such units are relatively simple and well developed and insulating double-pane units of this type are widely used. However, such double-pane units have not, to date, been applied to our knowledge in solar collectors nor have the techniques used in fabricating such units been applied to solve the problems encountered in the fabrication of solar collectors.
Mention should also be made of the fact that it is known, in the solar-energy-collector art, to provide the highest possible absorption capacity and minimum emission capability so as to make the collector efficiency as high as possible.
Maximum absorption capacity means that as much as possible of the incident solar energy should be absorbed and retained in the form of sensible heat in the solar collector while reduced emission capability implies that as little as possible of the incident solar energy should be re-radiated as heat or reflected from the collector.
To this end it is known to provide the solar-energy collector with two layers, namely, an emission-limiting layer on a surface of the solar collector turned toward the sun and a layer of high absorption capability preferably upon a surface turned away from the sun.
This is achieved in certain prior-art devices by providing the solar-energy collector with a collector plate composed of sheet metal which defines the flow chamber on the solar-energy-incident side of the device (see German open application-Offenlegungsschrift-No. DT-OS 26 15 686). It is also known to provide the flow chambers as glass cubes which are of the type described in German open application-Offenlegungsschrift-No. DT-OS 25 39 965.
Even these techniques have not, however, solved the major problems which have hitherto been encountered with solar-energy collectors with respect to maximizing the absorption efficiency and reducing re-emitted radiation.
As a practical matter it is found that most of the absorption takes place by the absorption layer so that the heat must be transferred by a wall of the collector carrying or constituting the absorption layer to the fluid traversing the flow chamber. As a result, the fluid does not function effectively to pick up solar energy directly and efficiency loss results from the need to provide an additional heat transfer at the interface between the absorption layer and the wall and at the interface between the wall and the fluid.
When the flow chambers of the solar-energy collector are constituted of metal or are defined between metal walls, considerable efficiency loss may also result from re-reflection and conductive dissipation of the solar energy.